random number

A number generated by a process that is fundamentally non-deterministic
and unpredictable. Computer-generated "random numbers," which are calculated
through a deterministic process, cannot, by definition, be random.
Given knowledge of the algorithm used to create the numbers and its internal
state, it is possible to predict all of the numbers returned by subsequent
calls to the algorithm. For this reason
the numbers produced by computer-based "random number generators" are often
referred to as pseudorandom numbers. In the case of genuinely
random numbers, knowledge of one number or an arbitrarily long sequence
of numbers offers no clue as to what the next number to be generated will
be. Humans are among the worst random number generators. Ask someone to
pick a number "at random" between 1 and 20, and the number they are most
likely to choose is 17. Psychologically random numbers, such as 17, are
usually odd and don't end in 5, so that they frequently tend to be prime
numbers.